1. Field of the Invention
This invention relates to a precure tread:for tire, pneumatic tires using the same and a method of producing such a pneumatic tire. Particularly, the invention provides a considerable improvement of tread durability when this precure tread is applied to, for example, a new or retreaded tire for truck and bus.
2. Related Art Statement
In tires, such as pneumatic tire for truck and bus and the like, as the running distance increases, a tread 2 of a tire 1 is gradually worn as shown in FIG. 19 to shallow the depth of circumferential main grooves 3 formed on the surface of the tread 2, whereby the road gripping force of the tread 2 as well as the wet performances of the tire greatly lower. Therefore, the tire 1 is generally and frequently retreaded at a state that the main groove 3 is left to a certain extent in the tread 2.
The retreading of the tire is usually carried out by buffing the remaining tread portion 2a of the worn tire 1 to form a base tire 4 as shown, for example, by dotted lines in FIG. 19, and then bonding a precure tread 6 to a crown portion 4a of the base tire 4 through a cushion rubber 5 by vulcanization as shown in FIG. 20.
Moreover, the retreading of the tire is repeated 2 to 3 times or may be repeated more than 3 times though it changes in accordance with the service conditions. In case of repeating the retreading, if the wear resistance of the tread is low or the wet performances and the like at last worn stage are low, the period between the retreading and next retreading becomes shorter and the retreading becomes uneconomical.
In order to sufficiently develop the wet performances and other performances of the tread even at the last worn stage, there is proposed a tire as described in Japanese Utility Model laid open No. 51-95,902.
In this proposed tire, a rear groove extending from a position separated outward from the bottom of the surface groove in the radial direction toward the inside in the radial direction is continuously or discontinuously disposed in the inside of the tread in the circumferential direction of the tire. In this tire, the rear groove is exposed at the last worn stage, whereby the degradation of the wet performances and other performances can effectively be prevented. That is, when this proposal is applied to a precure tread 8 of a retreaded tire 7 as shown in FIG. 21, the precure tread 8 comprises plural main grooves 3 at its ground contacting portion 8a and a rear groove 9 at its rear portion 8b facing the base tire. When the tread 8 is worn up to a position shown by dot-dash lines M in FIG. 21 during the use of the retreaded tire 7, the rear groove 9 appears at a tread face 8c, and consequently the drainage effect through the rear groove 9 as well as the edge effect at the groove edge of the rear groove are generated to largely improve the wet performances.
Furthermore, the precure tread 8 is designed to provide sufficient wet performances at the last worn stage, so that it is useless to ensure extra wet performances in the precured tread over a period ranging from initial worn stage to last worn stage. As a result, a ratio of surface groove area occupied in ground contacting area of the tread or so-called negative ratio can be made small to improve the wear resistance and resistance to uneven wear.
In the tire of FIG. 21, however, the rear groove 9 forms a closed space through the bonding of the precure tread 8 to the cushion rubber 10, so that gas existing in the rear groove 9 thermally expands through compressive deformation of the tread 8 repeated during the running of the tire and gets into an interface between the tread 8 and the cushion rubber 10 due to the increase of the volume of the gas itself. As a result, there is a problem of peeling a portion of the tread 8 near the rear groove 9 from the cushion rubber 10 as exaggeratedly shown in FIG. 21 to degrade the durability of the tire.